Estrogen influences brain development in females at puberty. Environmental and cultural factors interact with the biological effects of estrogen on the brain and consequently on cognition and behavior. Women with Turner syndrome lack endogenous estrogen as a result of dysgenetic ovaries. Turner syndrome, therefore, represents a unique, estrogen- deficient model in which to study the biological effects of estrogen on cognition and behavior. The specific aims of this project are to: 1) to examine the differential effects of continuous estrogen replacement in early childhood on cognitive and social function in an ongoing, unique, randomized, double-blind, placebo-controlled, treatment trial. 2) document further, the cognitive differences between girls with Turner syndrome at ages 8 and 12 years versus age-matched, normal girls. Specifically, we hypothesize that estrogen replacement in early childhood will reduce the cognitive deficits of girls with Turner syndrome. In addition, we hypothesize that earlier (age 5-7 years) and longer estrogen replacement will result in less impairment of visual-spacial ability, visual-motor ability, social function, and affective competence compared to later (9 to 12 years) estrogen replacement in girls with Turner syndrome. Finally, we hypothesize that the degree of social function in these girls will correlate with visual-spatial ability and facial recognition ability. The data generated from this carefully controlled biological investigation of cognitive and social development is an important stage in understanding normal brain development. In addition, these data will help determine how to optimize cognitive function in Turner syndrome, and will extend knowledge of the underlying mechanisms of sexual dimorphism.